Sodium-halogen secondary cell

ABSTRACT

An intermediate temperature sodium-halogen secondary cell that includes a negative electrode compartment housing a negative, molten sodium-based electrode and a positive electrode compartment housing a current collector disposed in a highly conductive molten positive electrolyte. A sodium halide (NaX) positive electrode is disposed in a molten positive electrolyte comprising one or more AlX 3  salts, wherein X may be the same or different halogen selected from Cl, Br, and I, wherein the ratio of NaX to AlX 3  is greater than or equal to one. A sodium ion conductive solid electrolyte membrane separates the molten sodium negative electrode from the molten positive electrolyte. The secondary cell operates at a temperature in the range from about 80° C. to 210° C.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. ProvisionalPatent Application Ser. No. 62/087,507 entitled “SODIUM-HALOGENSECONDARY CELL” filed Dec. 4, 2014. This application is also acontinuation-in-part of U.S. patent application Ser. No. 14/511,031,entitled “SODIUM-HALOGEN SECONDARY CELL,” filed Oct. 9, 2014, whichclaims the benefit of U.S. Provisional Patent Application Ser. No.61/888,933 entitled “NASICON MEMBRANE BASED Na—I₂ BATTERY,” filed Oct.9, 2013. This application is also a continuation-in-part of U.S. patentapplication Ser. No. 14/019,651, entitled “SODIUM-HALOGEN BATTERY,”filed Sep. 6, 2013, which claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/697,608 entitled “SODIUM-HALOGEN BATTERY,” filedSep. 6, 2012, and which also claims the benefit of U.S. ProvisionalPatent Application Ser. No. 61/777,967 entitled “SODIUM-HALOGENSECONDARY CELL,” filed Mar. 12, 2013, and which also claims the benefitof U.S. Provisional Patent Application Ser. No. 61/781,530 entitled“SODIUM-HALOGEN SECONDARY FLOW CELL,” filed Mar. 14, 2013, and whichalso claims the benefit of U.S. Provisional Patent Application Ser. No.61/736,444 entitled “BATTERY WITH BROMINE OR BROMIDE ELECTRODE ANDSODIUM SELECTIVE MEMBRANE,” filed Dec. 12, 2012. All of these priorpatent applications are expressly incorporated herein by reference.

STATEMENT REGARDING FEDERALLY FUNDED RESEARCH

This invention was made with government support under Contract No.1189875 awarded by the Sandia National Lab. The government has certainrights in the invention.

TECHNICAL FIELD

The disclosed invention relates to an intermediate temperature,sodium—halogen secondary cell (or rechargeable battery) with a sodiumion conductive electrolyte membrane and a positive electrolyte thatcomprises one or more sodium haloaluminate salts and a sodium halide. Insome disclosed embodiments, the battery system utilizes a molteneutectic mixture of sodium haloaluminate salts having a relatively lowmelting point.

BACKGROUND

Batteries are known devices that are used to store and releaseelectrical energy for a variety of uses. In order to produce electricalenergy, batteries typically convert chemical energy directly intoelectrical energy. Generally, a single battery includes one or moregalvanic cells, wherein each of the cells is made of two half-cells thatare electrically isolated except through an external circuit. Duringdischarge, electrochemical reduction occurs at the cell's positiveelectrode, while electrochemical oxidation occurs at the cell's negativeelectrode. While the positive electrode and the negative electrode inthe cell do not physically touch each other, they are generallychemically connected by at least one (or more) ionically conductive andelectrically insulative electrolytes, which can either be in a solidstate, a liquid state, or in a combination of such states. When anexternal circuit, or a load, is connected to a terminal that isconnected to the negative electrode and to a terminal that is connectedto the positive electrode, the battery drives electrons through theexternal circuit, while ions migrate through the electrolyte.

Batteries can be classified in a variety of manners. For example,batteries that are completely discharged only once are often referred toas primary batteries or primary cells. In contrast, batteries that canbe discharged and recharged more than once are often referred to assecondary batteries or secondary cells. The ability of a cell or batteryto be charged and discharged multiple times depends on the Faradaicefficiency of each charge and discharge cycle.

While rechargeable batteries based on sodium can comprise a variety ofmaterials and designs, most, if not all, sodium batteries that require ahigh Faradaic efficiency employ a solid primary electrolyte separator,such as a solid ceramic primary electrolyte membrane. The principaladvantage of using a solid ceramic primary electrolyte membrane is thatthe Faradaic efficiency of the resulting cell approaches 100%. Indeed,in almost all other cell designs, electrode solutions in the cell areable to intermix over time and, thereby, cause a drop in Faradaicefficiency and loss of battery capacity.

The primary electrolyte separators used in sodium batteries that requirea high Faradaic efficiency often consist of ionically conductivepolymers, porous materials infiltrated with ionically conductive liquidsor gels, or dense ceramics. In this regard, many rechargeable sodiumbatteries that are presently available for commercial applicationscomprise a molten sodium metal negative electrode, a sodium β″-aluminaceramic electrolyte separator, and a molten positive electrode, whichmay include a composite of molten sulfur and carbon (called asodium/sulfur cell). Because these conventional high temperaturesodium-based rechargeable batteries have relatively high specific energydensities and only modest power densities, such rechargeable batteriesare typically used in certain specialized applications that require highspecific energy densities where high power densities are typically notencountered, such as in stationary storage and uninterruptable powersupplies.

Despite the beneficial characteristics associated with some conventionalsodium-based rechargeable batteries, such batteries may have significantshortcomings. In one example, because the sodium β″-alumina ceramicelectrolyte separator is typically more conductive and is better wettedby molten sodium at a temperature in excess of about 270° C. and/orbecause the molten positive electrode typically requires relatively hightemperatures (e.g., temperatures above about 170° or 180° C.) to remainmolten, many conventional sodium-based rechargeable batteries operate attemperatures higher than about 270° C. and are subject to significantthermal management problems and thermal sealing issues. For example,some sodium-based rechargeable batteries may have difficulty dissipatingheat from the batteries or maintaining the negative electrode and thepositive electrode at the relatively high operating temperatures. Inanother example, the relatively high operating temperatures of somesodium-based batteries can create significant safety issues. In stillanother example, the relatively high operating temperatures of somesodium-based batteries require their components to be resistant to, andoperable at, such high temperatures. Accordingly, such components can berelatively expensive. In yet another example, because it may require arelatively large amount of energy to heat some conventional sodium-basedbatteries to the relatively high operating temperatures, such batteriescan be expensive to operate and energy inefficient.

Thus, while sodium-based rechargeable batteries are available,challenges with such batteries also exist, including those previouslymentioned. Accordingly, it would be an improvement in the art to augmentor even replace certain conventional sodium-based rechargeable batterieswith other sodium-based rechargeable batteries that operate effectivelyat intermediate temperatures.

SUMMARY OF THE INVENTION

Examples of sodium-halogen secondary cells are disclosed in Applicant'scopending U.S. patent application Ser. No. 14/019,651, published as U.S.Publication No. 2014/0065456 entitled “Sodium-Halogen Secondary Cell.”The disclosed secondary cells include a positive electrode compartmenthousing a current collector disposed in a liquid positive electrodesolution. Some examples of suitable positive electrode solutionmaterials include organic solvents such as dimethyl sulfoxide, NMF(N-methylformamide), and ionic liquids.

The present disclosure provides an improvement to the positive electrodesolution of the sodium-halogen secondary cells disclosed in Applicant'scopending application. More specifically, the disclosed inventionutilizes a positive electrolyte that comprises sodium halide in a moltenhaloaluminate electrolyte. In some disclosed embodiments, the batterysystem utilizes a molten eutectic mixture of sodium haloaluminate saltshaving a relatively low melting point.

A sodium ion conductive solid electrolyte separates the negativeelectrode and the positive electrode. In a non-limiting embodiment, thesodium ion conductive solid electrolyte comprises a NaSICON electrolytematerial. The NaSICON electrolyte material has high sodium conductivityat cell operating temperatures.

In one non-limiting embodiment, the battery operates at a temperature inthe range from 80° C. to 210° C.

In one non-limiting embodiment of the disclosed invention, therechargeable sodium-halogen battery includes a negative electrodecomprising metallic sodium in molten state. In another embodiment, thenegative electrode may comprise metallic sodium in a solid state. Thepositive electrode comprises NaX, where X is a halogen selected from Cl,Br and I. The positive electrode is disposed in a molten salt positiveelectrolyte comprising AlX₃. In some embodiments, the positiveelectrolyte is a mixture of at least two AlX₃ salts that can berepresented by the formula NaAlX′_(4−δ)X″_(δ), where 0<δ<4, wherein X′and X″ are different halogens selected from Cl, Br and I.

The mixed molten salt positive electrolyte comprises at least two saltsof the general formula NaAlX′₄ and NaAlX″₄ at various molar ratios,wherein X′ and X″ are different halogens selected from Cl, Br and I. Inone non-limiting embodiment, the molar ratio of NaAlX′₄ to NaAlX″₄ is inthe range of 9:1 to 1:9 with corresponding δ values of 0.4 to 3.6.

The positive electrode comprises additional NaX or a mixture of NaXcompounds added in a molar ratio to the mixed molten salt positiveelectrolyte ranging from 1:1 to 3:1 of NaX:NaAlX′_(4−δ)X″_(δ). Theexcess NaX renders the positive electrolyte highly basic. At celloperating temperatures, the positive electrode and mixed molten saltpositive electrolyte is a molten liquid or a two phase mixture whereinthe mixed molten salt positive electrolyte is predominantly a liquidphase and the additional NaX or mixture of NaX compounds is a solidphase.

In other embodiments, the positive electrode is disposed in a mixedmolten salt positive electrolyte comprising at least three salts thatcan be represented by the formula NaAlX′_(4−δ−ω) X″_(δ)X′″ _(ω) , whereX′, X″ and X′″ are three different halogens selected from Cl, Br, and I,where 0<δ<4, 0<ω<4, and 0<δ+ω<4. The mixed molten salt positiveelectrolyte comprises NaAlCl₄, NaAlBr₄, and NaAlI₄, at various molarratios.

The disclosed sodium haloaluminate molten salts are highly conductive atrelatively low temperatures enabling the sodium-halogen battery to behighly efficient and reversible. These features and advantages of thepresent embodiments will become more fully apparent from the followingdescription and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the manner in which the above-recited and other featuresand advantages of the invention are obtained will be readily understood,a more particular description of the invention briefly described abovewill be rendered by reference to specific embodiments thereof that areillustrated in the appended drawings. Understanding that these drawingsdepict only typical embodiments of the invention and are not thereforeto be considered to be limiting of its scope, the invention will bedescribed and explained with additional specificity and detail throughthe use of the accompanying drawings in which:

FIG. 1 depicts a schematic diagram of a representative embodiment of amolten sodium-halogen secondary cell, wherein the cell is in the processof being discharged.

FIG. 2 depicts a schematic diagram of a representative embodiment of themolten sodium-halogen secondary cell, wherein the cell is in the processof being recharged.

FIG. 3A depicts one potential reaction, designated Battery Chemistry 1,at the positive current collector.

FIG. 3B depicts another potential reaction, designated Battery Chemistry2, at the positive current collector.

FIG. 4 is a graph comparing the conductivity of NaI in a molten saltelectrolyte and in an organic solvent as a function of temperature.

FIGS. 5A and 5B are graphs comparing the oxidation of iodide in asodium-iodine secondary cell containing NaI in AlCl₃ at basic and acidicratios of NaI:AlCl₃.

FIG. 6 is a graph of the current vs. voltage for the operation of thesymmetrical cells described in Example 3.

DETAILED DESCRIPTION

Reference throughout this specification to “one embodiment,” “anembodiment,” or similar language means that a particular feature,structure, or characteristic described in connection with the embodimentis included in at least one embodiment of the present invention. Thus,appearances of the phrases “in one embodiment,” “in an embodiment,” “inanother embodiment,” and similar language throughout this specificationmay, but do not necessarily, all refer to the same embodiment.Additionally, while the following description refers to severalembodiments and examples of the various components and aspects of thedescribed invention, all of the described embodiments and examples areto be considered, in all respects, as illustrative only and not as beinglimiting in any manner.

Furthermore, the described features, structures, or characteristics ofthe invention may be combined in any suitable manner in one or moreembodiments. In the following description, numerous specific details areprovided, such as examples of suitable sodium-based negative electrodes,liquid positive electrode solutions, current collectors, sodium ionconductive electrolyte membranes, etc., to provide a thoroughunderstanding of embodiments of the invention. One having ordinary skillin the relevant art will recognize, however, that the invention may bepracticed without one or more of the specific details, or with othermethods, components, materials, and so forth. In other embodiments,well-known structures, materials, or operations are not shown ordescribed in detail to avoid obscuring aspects of the invention.

As stated above, secondary cells can be discharged and recharged andthis specification describes cell arrangements and methods for bothstates. Although the term “recharging” in its various forms implies asecond charging, one of skill in the art will understand thatdiscussions regarding recharging would be valid for, and applicable to,the first or initial charge, and vice versa. Thus, for the purposes ofthis specification, the terms “recharge,” “recharged,” and“rechargeable” shall be interchangeable with the terms “charge,”“charged,” and “chargeable,” respectively.

The present embodiments provide a sodium-halogen secondary cell, whichincludes a molten or solid state sodium negative electrode and a sodiumhalide positive electrode disposed in a molten positive electrolyte thatcomprises one or more haloaluminate salts. In some disclosedembodiments, the secondary cell utilizes a molten eutectic mixture ofsodium haloaluminate salts having a relatively low melting point.Although the described cell can comprise any suitable component, FIG. 1shows a representative embodiment in which the sodium secondary cell 10comprises a negative electrode compartment 15 that includes a sodiummetal negative electrode 20 and a positive electrode compartment 25 thatcomprises a sodium halide positive electrode. The positive electrodeincludes a current collector 30 disposed in a positive electrolyte 35comprising one or more molten haloaluminate salts (AlCl₃, AlBr₃, andAlI₃). A sodium ion conductive electrolyte membrane 40 separates thenegative electrode from the positive electrode and positive electrolyte35. The sodium ion conductive electrolyte membrane 40 separates a firstterminal 45 from a second terminal 50. To provide a better understandingof the described cell 10, a brief description of how the cell functionsis provided below. Following this discussion, each of the cell'scomponents shown in FIG. 1 is discussed in more detail.

Turning now to the manner in which the sodium secondary cell 10functions, the cell can function in virtually any suitable manner. Inone example, FIG. 1 illustrates that as the cell 10 is discharged andelectrons (e⁻) flow from the negative electrode 20 (e.g., via the firstterminal 45), sodium is oxidized from the negative electrode 20 to formsodium ions (Na⁺). FIG. 1 shows that these sodium ions are respectivelytransported from the sodium negative electrode 20, through the sodiumion conductive membrane 40, and to the positive electrolyte 35.

In a contrasting example, FIG. 2 shows that as the secondary cell 10 isrecharged and electrons (e⁻) flow into the sodium negative electrode 20from an external power source (not shown), such as a recharger, thechemical reactions that occurred when the cell 10 was discharged (asshown in FIG. 1) are reversed. Specifically, FIG. 2 shows that as thecell 10 is recharged, sodium ions (Na⁺) are respectively transportedfrom the positive electrolyte 35, through the electrolyte membrane 40,and to the negative electrode 20, where the sodium ions are reduced toform sodium metal (Na).

Referring now to the various components of the cell 10, the cell, asmentioned above, can comprise a negative electrode compartment 15 and apositive electrode compartment 25. In this regard, the two compartmentscan be any suitable shape and have any other suitable characteristicthat allows the cell 10 to function as intended. By way of example, thenegative electrode and the positive electrode compartments can betubular, rectangular, or be any other suitable shape. Furthermore, thetwo compartments can have any suitable spatial relationship with respectto each other. For instance, while FIG. 2 shows that the negativeelectrode compartment 15 and the positive electrode compartment 25 canbe adjacent to each other, in other embodiments (not shown), onecompartment (e.g., the negative electrode compartment) is disposed, atleast partially, in the other compartment (e.g., the positive electrodecompartment), while the contents of the two compartments remainseparated by the electrolyte membrane 40 and any other compartmentalwalls.

With respect to the negative electrode 20, the cell 10 can comprise anysuitable sodium negative electrode 20 that allows the cell 10 tofunction (e.g., be discharged and recharged) as intended. Some examplesof suitable sodium negative electrode materials include, but are notlimited to, a sodium sample that is substantially pure and a sodiumalloy comprising any other suitable sodium-containing negative electrodematerial. In certain embodiments, however, the negative electrodecomprises or consists of an amount of sodium that is substantially pure.In such embodiments, because the melting point of pure sodium is around98° C., the sodium negative electrode will become molten above thattemperature.

With respect to the positive current collector 30, the positiveelectrode compartment 25 can comprise any suitable positive electrodethat allows the cell to be charged and discharged as intended. Forinstance, the positive electrode can comprise virtually any currentcollector 30 in combination with a halogen, shown generically as “X” inFIGS. 1 and 2, in a positive electrolyte 35 comprising one or moresodium haloaluminate salts. The current collector 30 can be disposed inany suitable location in the positive electrode compartment 25 thatallows the cell 10 to function as intended.

With respect to the current collector 30, the cell 10 can comprise anysuitable current collector that allows the cell to be charged anddischarged as intended. For instance, the current collector can comprisevirtually any current collector configuration that has been successfullyused in a sodium-based rechargeable battery system. In some embodiments,the current collector comprises at least one of wires, felts, foils,plates, parallel plates, tubes, meshes, mesh screens, foams, and/orother suitable current collector configuration. It will be appreciatedby those of skill in the art that the foam may include, withoutlimitation, metal foams and carbon foams. Indeed, in some embodiments,the current collector comprises a configuration having a relativelylarge surface area which may include one or more mesh screens and metalfoams.

The current collector 30 can comprise any suitable material that allowsthe cell 10 to function as intended. In this regard, some non-limitingexamples of suitable current collector materials include tungsten,stainless steel, carbon, molybdenum, titanium, platinum, copper, nickel,zinc, a sodium intercalation material (e.g., Na_(x)MnO₂, etc.), nickelfoam, nickel, a sulfur composite, a sulfur halide (e.g., sulfuricchloride), and/or another suitable material. Furthermore, thesematerials may coexist or exist in combinations. In some embodiments,however, the current collector comprises tungsten, carbon, molybdenum,titanium.

In some non-limiting embodiments, the reactions that may occur at thenegative electrode 20, the positive electrode/current collector 30, andthe overall reaction as the cell 10 is discharged may occur in at leasttwo steps. These two potential reactions are shown below and designatedBattery Chemistry 1 (shown schematically in FIG. 3A for batteryrecharge) and Battery Chemistry 2 (shown schematically in FIG. 3B forbattery recharge). It has been observed that these reactions may beindividual steps of a multi-step reaction, or depending upon the batteryconditions, one step may be favored over another step.

-   -   Negative electrode Na        Na⁺+1e⁻    -   Positive electrode X₃ ⁻+2e⁻        3X⁻ (Battery Chemistry 1)    -   Positive electrode 3X₂+2e⁻        2X₃ ⁻ (Battery Chemistry 2)    -   Overall 2Na+X₃ ⁻        2Na⁺+3X⁻ (Battery Chemistry 1)    -   Overall 2Na+3X₂        2Na⁺+2X₃ ⁻ (Battery Chemistry 2)

Where X comprises iodine, bromine, or chlorine.

Where X comprises iodine, the cell 10 may have the following chemicalreactions and the following theoretical voltage (V vs. SHE (standardhydrogen electrode)) and theoretical specific energy (Wh/kg):

-   -   Negative electrode Na        Na⁺+1e⁻ (−2.71V)    -   Positive electrode I₃ ⁻+2e⁻        3I⁻ (0.29V, Chemistry 1)    -   Positive electrode 3I₂+2e⁻        2I₃ ⁻ (0.74V, Chemistry 2)    -   Overall 2Na+I₃ ⁻        2Na⁺+3I⁻ (2.8V, Chemistry 1) (388 Wh/kg)    -   Overall 2Na+3I₂        2Na⁺+2I₃ ⁻ (3.25V, Chemistry 2) (193 Wh/kg)

Where X is iodine, the charging reactions at the positive electrode mayoccur in two steps: 1) iodide to triiodide and 2) triiodide to iodine.Similarly, discharging reactions at the positive electrode may occur intwo steps: 1) iodine to triiodide and 2) triiodide to iodide.Alternatively, the charging and discharging reactions may occur usingthe combination of reaction chemistries above.

Where X is bromine, the cell 10 may have the following chemicalreactions and the following theoretical voltage (V vs. SHE) andtheoretical specific energy (Wh/kg):

-   -   Negative electrode Na        Na⁺+1e⁻ (−2.71V)    -   Positive electrode Br₃ ⁻+2e⁻        3Br⁻ (0.82V, Chemistry 1)    -   Positive electrode 3Br₂+2e⁻        2Br₃ ⁻ (1.04V, Chemistry 2)    -   Overall 2Na+Br₃ ⁻        2Na⁺+3Br⁻ (3.53V, Chemistry 1) (658 Wh/kg)    -   Overall 2Na+3Br₂        2Na⁺+2Br₃ ⁻ (3.75V, Chemistry 2) (329 Wh/kg)

The charging reactions at the positive electrode may occur in twosteps: 1) bromide to tribromide and 2) tribromide to bromine. Similarly,discharging reactions at the positive electrode may occur in twosteps: 1) bromine to tribromide and 2) tribromide to bromide.Alternatively, the charging and discharging reactions may occur usingthe combination of reaction chemistries above.

It will be appreciated by those of skill in the art that an alternativepositive electrode chemistry may include:

-   -   Positive electrode X₂+2e⁻        2X⁻ (Battery Chemistry 3)

With an overall battery chemistry of:

-   -   Overall 2Na+X₂        2Na⁺+2X⁻ (Battery Chemistry 3)

With regards now to the sodium ion conductive electrolyte membrane 40,the membrane can comprise any suitable material that selectivelytransports sodium ions and permits the cell 10 to function with apositive electrolyte 35. In some embodiments, the electrolyte membranecomprises a NaSICON-type (sodium Super Ion CONductive) material. Wherethe electrolyte membrane comprises a NaSICON-type material, theNaSICON-type material may comprise any known or novel NaSICON-typematerial that is suitable for use with the described cell 10. Somesuitable examples of NaSICON-type compositions include, but are notlimited to, Na₃Zr₂Si₂PO₁₂, Na_(1+x)Si_(x)Zr₂P_(3−x)O₁₂ (where x isbetween about 1.6 and about 2.4), Y-doped NaSICON(Na_(1+x+y)Zr_(2−y)Y_(y)Si_(x)P_(3−x)O₁₂,Na_(1+x)Zr_(2−y)Y_(y)Si_(x)P_(3−x)O_(12−y) (where x=2, y=0.12)),Na_(1−x)Zr₂Si_(x)P_(3−x)O₁₂ (where x is between about 0 and about 3, andin some cases between about 2 and about 2.5), and Fe-doped NaSICON(Na₃Zr₂/₃Fe₄/₃P₃O₁₂). Indeed, in certain embodiments, the NaSICON-typemembrane comprises Na₃Si₂Zr₂PO₁₂. In other embodiments, the NaSICON-typemembrane comprises one or more NaSELECT® materials, produced byCeramatec, Inc. in Salt Lake City, Utah.

The positive electrode comprises NaX, where X is a halogen selected fromCl, Br and I. The positive electrode is preferably NaI.

The positive electrode is disposed in a molten salt positive electrolytecomprising AlX₃. NaX and AlX₃ may combine to form NaAlX₄ as follows:

-   -   NaX+AlX₃        AlX₄

In some embodiments, the positive electrode is combined with a mixtureof at least two AlX₃ salts. The combination of positive electrode andpositive electrolyte can be represented by the general formulaNaAlX′_(4−δ)X″_(δ), where 0<δ<4, wherein X′ and X″ are differenthalogens selected from Cl, Br and I.

The mixed molten salt positive electrolyte comprises at least two saltsof the general formula NaAlX′₄ and NaAlX″₄ at various molar ratios,wherein X′ and X″ are different halogens selected from Cl, Br and I. Inone non-limiting embodiment, the molar ratio of NaAlX′₄ to NaAlX″₄ is inthe range of 9:1 to 1:9 with corresponding δ values of 0.4 to 3.6.

The positive electrode comprises additional NaX or a mixture of NaXcompounds added in a molar ratio to the mixed molten salt positiveelectrolyte ranging from 1:1 to 3:1 of NaX:NaAlX′_(4−δ)X″_(δ). Theexcess NaX renders the positive electrolyte highly basic. At celloperating temperatures, the positive electrode and mixed molten saltpositive electrolyte is a molten liquid or a two phase mixture whereinthe mixed molten salt positive electrolyte is predominantly a liquidphase and the additional NaX or mixture of NaX compounds is a solidphase.

The following Table 1 illustrates some non-limiting combinations of NaXand AlX₃ to form NaAlX₄.

TABLE 1 AlX₃ NaX AlCl₃ AlBr₃ AlI₃ NaCl NaAlCl₄ NaAlBr₃Cl NaAlI₃Cl NaBrNaAlCl₃Br NaAlBr₄ NaAlI₃Br NaI NaAlCl₃I NaAlBr₃I NaAlI₄

In other embodiments, the positive electrode is disposed in a mixedmolten salt positive electrolyte comprising at least three salts thatcan be represented by the formula NaAlX′_(4−δ−ω) X″_(δ)X′″ _(ω) , whereX′, X″ and X′″ are three different halogens selected from Cl, Br, and I,where 0<δ<4, 0<ω<4, and 0<δ+ω<4. The mixed molten salt positiveelectrolyte comprises NaAlCl₄, NaAlBr₄, and NaAlI₄, at various molarratios.

In some embodiments, the positive electrolyte 35 also comprises one ormore halogens and/or halides. In this regard, the halogens and halides,as well polyhalides and/or metal halides that form therefrom (e.g.,where the current collector 30 comprises a metal, such as copper,nickel, zinc, etc. (as discussed below)) can perform any suitablefunction, including, without limitation, acting as the positiveelectrode as the cell 10 operates. Some examples of suitable halogensinclude bromine, iodine, and chlorine. Similarly, some examples ofsuitable halides include bromide ions, polybromide ions, iodide ions,polyiodide ions, chloride ions, and polychloride ions. While thehalogens/halides can be introduced into the positive electrode solutionin any suitable manner, in some embodiments, they are added as NaX,wherein X is selected from Br, I, Cl, etc.

With reference now to the terminals 45 and 50, the cell 10 can compriseany suitable terminals that are capable of electrically connecting thecell with an external circuit (not shown), including without limitation,to one or more cells. In this regard, the terminals can comprise anysuitable material, be of any suitable shape, and be of any suitablesize.

In addition to the aforementioned components, the cell 10 can optionallycomprise any other suitable component. By way of non-limitingillustration FIGS. 1 and 2 show an embodiment in which the cell 10comprises a heat management system 55, 60. Independent heat managementsystems may be associated with the negative electrode and positiveelectrode compartments. Alternatively, a single heat management systemmay be disposed in only one compartment or to the exterior of the cell10 generally. In such embodiments, the cell can comprise any suitabletype of heat management system that is capable of maintaining the cellwithin a suitable operating temperature range. Some examples of suchheat management systems include, but are not limited to, a heater, acooler, one or more temperature sensors, and appropriate temperaturecontrol circuitry.

The described cell 10 may function at any suitable operatingtemperature. In other words, as the cell is discharged and/or recharged,the sodium negative electrode and the positive electrolyte may have anysuitable temperature. The negative and positive electrode compartmentsmay operate at the same or different temperatures. Indeed, in someembodiments, the cell functions at an intermediate operating temperaturein the range from about 80° C. to about 210° C. In other embodiments,the cell may function at an intermediate operating temperature in therange from about 110° C. to about 180° C. In yet another embodiment, theoperating temperature of the cell in the range of about 150° C. to about170° C.

The following examples are given to illustrate various embodimentswithin, and aspects of, the scope of the present invention. These aregiven by way of example only, and it is understood that the followingexamples are not comprehensive or exhaustive of the many types ofembodiments of the present invention that can be prepared in accordancewith the present invention.

Example 1

The conductivity of NaI in a molten salt positive electrolyte AlCl₃ wascompared to the conductivity of NaI in an organic solvent solution thatincluded N-methyl formamide. The molten salt positive electrolyte had ageneral formula of NaAl_(x)I_(y)Cl_(z). The conductivity of NaI in amolten salt positive electrolyte was approximately three times theconductivity of the organic solvent-based electrolyte at 120° C., asshown in FIG. 4. Cells utilizing a molten salt positive electrolyte willbe more energy dense due to higher molarity of NaI. Furthermore, cellsutilizing a molten salt positive electrolyte are safer than organicsolvent based positive electrolyte solutions because if molten sodiumhappens to contact the molten salt positive electrolyte, the chemicalreaction would only produce non-flammable salts.

Example 2

A sodium-iodine secondary cell was prepared as described hereincontaining sodium iodide in molten AlCl₃ in a 60:40 NaI:AlCl₃ ratio (a“basic” electrolyte). Tungsten wire was used as the positive currentcollector. NaSICON was used to separate a molten sodium negativeelectrode from the positive electrode/positive electrolyte. Theoxidation of iodide was measured and found to produce two oxidationpeaks, consistent with Battery Chemistry 1 and Battery Chemistry 2,described herein. Experimental results are shown in FIG. 5A. Theoxidation peaks were found to be reversible. Additional tests wereperformed using an “acidic” electrolyte comprising sodium iodide inmolten AlCl₃ in a 40:60 NaI:AlCl₃ ratio. Experimental results are shownin FIG. 5B. The results suggest that the second oxidation peak in thebasic electrolyte occurs at a similar potential as the first iodideoxidation peak in the acidic electrolyte. The reduction peak in acidicelectrolyte occurs at a higher potential. This suggests that whether theelectrolyte is acidic or basic affects the potential of I₂ generation.

Example 3

Three symmetrical sodium-iodine secondary cells were prepared to testthe reversibility of the oxidation/reduction reactions that occur in thepositive electrode/positive electrolyte. The symmetrical cells wereprepared as set forth in Table 2, below:

TABLE 2 Electrode Electrolyte Cell Outside of tube Inside of tubeOutside of Tube Inside of Tube Symmetrical Cell 1 Graphite felt Graphitefelt 1M I₂ in 51:49 mol % 1M I₂ in 51:49 mol % with tungsten withtungsten NaI:AlCl₃ NaI:AlCl₃ wire wire Symmetrical Cell 2 0.93 gTungsten wire 60:40 mol % NaI:AlCl₃ 0.02475 g I₂ in NaI/0.93 g withgraphite 51:49 mol % NaI:AlCl₃ C/PTFE felt around tungsten meshSymmetrical Cell 3 Graphite felt 0.25″ Carbon 60:40 mol % NaI:AlCl₃50:50 mol % NaI:AlCl₃ with tungsten Rod wire

The symmetrical cells were operated as set forth in Table 3, below:

TABLE 3 Operating Range Upper Lower Current Temper- EIS Voltage VoltageDensity ature Ohmic System Limit Limit (mA/ Cell (° C.) (Ω) (Ω) (V) (V)cm²) Symmetrical 125 2.17 5.19 0.5 −0.5 91 Cell 1 Symmetrical 125 3.6016.40 0.5 −0.5 72 Cell 2 Symmetrical 125 5.00 6.70 0.15 −0.15 20 Cell 3

A graph of the current vs. voltage for the operation of the symmetricalcells is shown in FIG. 6. Because there is little or no hysteresis shownin FIG. 6, it may be concluded that the oxidation/reduction reactionsthat occur in the positive electrode/positive electrolyte are highlyreversible.

Embodiments of the present invention may be embodied in other specificforms without departing from its spirit or essential characteristics.The described embodiments and examples are to be considered in allrespects only as illustrative and not as restrictive. The scope of theinvention is, therefore, indicated by the appended claims rather than bythe foregoing description. All changes that come within the meaning andrange of equivalency of the claims are to be embraced within theirscope.

What is claimed is:
 1. A sodium-halogen secondary cell, comprising: anegative electrode compartment comprising a negative electrode thatcomprises metallic sodium in molten or solid state, wherein the negativeelectrode electrochemically oxidizes to release sodium ions duringdischarge and electrochemically reduces sodium ions to form sodium metalduring recharge; a positive electrode compartment consisting of: a NaXpositive electrode disposed in a mixed molten positive electrolyteconsisting of one or more NaAlX₄ salts, wherein X is the same ordifferent element selected from Cl, Br, and I; additional NaX or amixture of NaX compounds present in a molar ratio of the additional NaXor mixture of NaX compounds to the mixed molten positive electrolyte of1:1 to 3:1; and a current collector; and a sodium ion conductive solidelectrolyte membrane comprising a NaSICON-type material and thatseparates the negative electrode compartment from the positive electrodecompartment; and wherein: the overall battery chemistry of thesodium-halogen secondary cell is 2Na+X₂↔2Na⁺+2X⁻; and the additional NaXor mixture of NaX compounds are present in a solid phase at operatingtemperature of the sodium-halogen secondary cell.
 2. The secondary cellof claim 1, wherein the mixed molten positive electrolyte is a mixtureof two different NaAlX₄ salts represented by the general formulaNaAlX′_(4−δ)X″_(δ), where 0<δ<4, wherein X′ and X″ are differentelements selected from Cl, Br and I.
 3. The secondary cell of claim 2,wherein the two different NaAlX₄ salts have the general formula NaAlX′₄and NaAlX″₄ at various molar ratios.
 4. The secondary cell of claim 3,wherein the molar ratio of NaAlX′₄ to NaAlX″₄ is in the range of 9:1 to1:9 with corresponding δ values of 0.4 to 3.6.
 5. The secondary cell ofclaim 3, wherein the molar ratio of the additional NaX or mixture of NaXcompounds to the mixed molten positive electrolyte is greater than 1:1to 3:1 of NaX:NaAlX′_(4−δ)X″_(δ).
 6. The secondary cell of claim 1,wherein the secondary cell operates at a temperature between 80° C. and210° C.
 7. The secondary cell of claim 1, wherein the mixed moltenpositive electrolyte is a mixture of three different NaAlX₄ salts and isrepresented by the general formula NaAlX′_(4−δ−ω) X″_(δ)X′″ _(ω) , whereX′, X″ and X′″ are three different elements selected from Cl, Br, and I,where 0<δ<4, 0<ω<4, and 0<δ+ω<4.
 8. The secondary cell of claim 7,wherein the three different NaAlX₄ salts have the formula NaAlCl₄,NaAlBr₄, and NaAI₄, at various molar ratios.
 9. The secondary cell ofclaim 7, wherein the additional NaX or mixture of NaX compounds arepresent in a molar ratio of the additional NaX or mixture of NaXcompounds to the mixed molten positive electrolyte ranging from greaterthan 1:1 to 3:1 of NaX:NaAlX′_(4−δ−ω) X″_(δ)X′″ _(ω) , where 0<δ<4,0<ω<4, and 0<δ+ω<4.
 10. The secondary cell of claim 1, wherein thecurrent collector comprises at least one of carbon, tungsten,molybdenum, and titanium.
 11. The secondary cell of claim 1, wherein thecurrent collector comprises at least one of wires, felts, foils, plates,parallel plates, tubes, meshes, mesh screens, and foams.
 12. Asodium-halogen secondary cell, comprising: a negative electrodecompartment comprising a negative electrode that comprises metallicsodium in molten state, wherein the negative electrode electrochemicallyoxidizes to release sodium ions during discharge and electrochemicallyreduces sodium ions to form sodium metal during recharge; a positiveelectrode compartment consisting of: a NaI positive electrode disposedin a mixed molten positive electrolyte consisting of comprising one ormore NaAlX₄ salts, wherein X is the same or different element selectedfrom Cl, Br, and I; additional NaI is present in a molar ratio of NaI tothe mixed molten positive electrolyte of 1:1 to 3:1; and a currentcollector; and a sodium ion conductive solid electrolyte membrane thatseparates the negative electrode compartment from the positive electrodecompartment; and wherein the overall battery chemistry of thesodium-halogen secondary cell is 2Na+I₂ ↔2Na⁺+2I⁻; and the additionalNaI is present in a solid phase at operating temperature of thesodium-halogen secondary cell.
 13. A sodium-halogen secondary cell,comprising: a negative electrode compartment comprising a negativeelectrode that comprises metallic sodium in molten or solid state,wherein the negative electrode electrochemically oxidizes to releasesodium ions during discharge and electrochemically reduces sodium ionsto form sodium metal during recharge; a positive electrode compartmentconsisting of: a NaX positive electrode disposed in a mixed moltenpositive electrolyte consisting of at least two different NaAlX₄ saltsand is represented by the general formula NaAlX′_(4−δ)X″_(δ), wherein0<δ<4, and X′ and X″ are different element selected from Cl, Br, and I;additional NaX or a mixture of NaX compounds present in a molar ratio ofthe additional NaX or mixture of NaX compounds to the mixed moltenpositive electrolyte ranging from greater than 1:1 to 3:1 ofNaX:NaAlX′_(4−δ)X″_(δ); and a current collector; and a sodium ionconductive solid electrolyte membrane that separates the negativeelectrode compartment from the positive electrode compartment; andwherein: the overall battery chemistry of the sodium-halogen secondarycell is 2Na+X₂↔2Na⁺+2X⁻; and the additional NaX or mixture of NaXcompounds are present in a solid phase within the positive electrode atoperating temperature of the sodium-halogen secondary cell.
 14. Thesecondary cell of claim 13, wherein the mixed molten positiveelectrolyte comprises three different NaAlX₄ salts and is represented bythe general formula NaAlX′_(4−δ−ω) X″_(δ)X′″ _(ω) , where X′, X″ and X′″are three different elements selected from Cl, Br, and I, where 0<δ<4,0<ω<4, and 0<δ+ω<4; and wherein the additional NaX or a mixture of NaXcompounds present in a molar ratio of the additional NaX or mixture ofNaX compounds to the mixed molten positive electrolyte ranging from 1:1to 3:1 of NaX:NaAlX′_(4−δ−ω) X″_(δ)X′″ _(ω) .
 15. The secondary cell ofclaim 14, wherein the three NaAlX₄ salts have the formula NaAlCl₄,NaAlBr₄, and NaAlI₄, at various molar ratios.
 16. The secondary cell ofclaim 13, wherein the secondary cell operates at a temperature betweenabout 80° C. and 210° C.
 17. The secondary cell of claim 13, wherein theelectrolyte membrane comprises a NaSICON-type material.